Once data has been written on a disk it is usually necessary to read that data at a later time. When this data has been written on a recording medium, for example, a hard disk drive, it is often necessary to correctly and accurately detect where this information is located. In part, this is accomplished by specifying the track number as part of the servo information. The track number alone is insufficient to completely specify the exact location of the data. In order to more closely identify the location of the data, additional servo information, which provides track center location information, is recorded on the surface of the recording medium.
FIG. 2 illustrates a portion of data sector 100 that has previously been recorded on a recording medium, for example, a hard disk drive. The data sector may include a synchronization field (sync field) 110 and a preamble 120 and user information or sector ID information 105. The synchronization field 110 may immediately follow the preamble 120 at the beginning of each data sector on a recording medium. The preamble 120 may be written with a 1/4T.sub.s squarewave of write current, where T.sub.s is the sample period. The preamble 120 may be read back as a high SNR sine wave, with inter-symbol interference (ISI), with phase-locked loop (PLL) and automatic gain control (AGC) circuitry to be used for timing and gain acquisition, respectively. The synchronization field 110, or sync field, indicates the beginning of the data for example, the user data or sector ID information.
The sync field pattern in the sync field 110 may be selected to minimize the probabilities of false alarm (false detection) of the sync field pattern and missed detection of the sync field pattern for the particular architecture of the sync field detector. For a given architecture, these probabilities may be determined by an operating bit error rate (BER) in the region around the sync field, the threshold of the number of comparison errors which will be accepted, and the Hamming distance of the sync field pattern from the preamble, from the sync field pattern itself and from data.
Sector Header Layout
FIG. 1 illustrates the layout of a typical sector as servo information 110 followed by a data sector 120. The data sector may include a data field 130 and an ID field 140. User data may be split across servo sectors. The components of the servo information may include a servo preamble which provides a pattern for PLL and/or AGC acquisition. The address mark follows the preamble. Peak detection circuitry determines the address mark, which functions like the sync field to indicate that the track number is to be read. Four servo bursts (additional or fewer servo bursts may be present) are evaluated and provide for calculating track position adjustments. This implies that during the header sections shown in FIG. 1, which may include preamble and sync field, the reading element may still be settling on track. The head's position off of the track's centerline will reduce the effective SNR and hence increase the BER in these fields.
The data field 130 of the data sector is shown in FIG. 2. The data preamble pattern may be shown as element 135, for example, a 1/4T.sub.s write current waveform, where T.sub.s is the sampling period of the pattern. This pattern, when read, provides a signal similar to a sinusoid with good SNR and well controlled ISI. The pattern may be used for PLL and/or AGC acquisition. The preamble may be up to 15 bytes long with each byte that may be 9 bits, although 8-bit bytes may be used. This preamble is followed by the sync field 137 typically consisting of fewer bits than the preamble, for example 18 bits long. Following the sync field there may be the user data plus Error Correction Coding (ECC) 139 which may include, for example 512K user bytes. These bits are constrained typically by the requirements of RLL (Run-Length Limited) encoding, for example 8/9 (0,4,4).